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Glossary - M
maintenance factor (MF)† a factor formerly used to denote the ratio of the illuminance on a given area after a period of time to the initial illuminance on the same area. This term is obsolete and is no longer valid. See light loss factor. matte surface a surface from which the reflection is predominantly diffuse, with or without a negligible specular component. See diffuse reflection. mean horizontal intensity (candlepower) the average intensity (in candelas) of a lamp in a plane perpendicular to the axis of the lamp that passes through the luminous center of the lamp. mean spherical luminous intensity the average value of the luminous intensity in all directions for a source. Also, the quotient of the total emitted luminous flux of the source by 4n. mean zonal candlepower the average intensity (candelas) of a symmetrical luminaire or lamp at an angle to the luminaire or lamp axis that is in the middle of the zone under consideration. means of egress† An unobstructed and continuous way of exit from any point in a building or structure to a public way. It consists of three distinct parts: the exit access, the exit, and the exit discharge. A means of egress consists of the vertical and horizontal travel ways including intervening room spaces, doorways, hallways, corridors, passageways, ramps, stairs, lobbies, horizontal exits, escalators, enclosures, courts, balconies, and yards. mercury lamp a high-intensity discharge (HID) lamp in which the major portion of the light is produced by radiation from mercury operating at a partial pressure in excess of 105 Pa (approximately 1 atm). Includes clear, phosphor-coated (mercury-fluorescent), and self-ballasted lamps. mercury-fluorescent lamp (phosphor mercury lamp) an electric-discharge lamp having a high- pressure mercury arc in an arc tube and an outer envelope coated with a fluorescing substance (phosphor) that transforms some of the ultraviolet energy generated by the arc into light. mesopic vision vision with fully adapted eyes at luminance conditions between those of photopic and scotopic vision, that is, between about 3.4 and 0.034 cd/m2. metal halide lamp a high-intensity discharge (HID) lamp in which the major portion of the light is produced by radiation of metal halides and their products of dissociation--possibly in combination with metallic vapors such as mercury. Includes clear and phosphor-coated lamps. metamers lights of the same color but of different spectral power distribution. Note The term "metamers" is also used to denote objects that, when illuminated by a given source and viewed by a given observer, produce metameric lights. middle ultraviolet† a portion of the electromagnetic spectrum in the range of 200 to 300 nm. minimal perceptible erythema, MPE the erythemal threshold. mired† See reciprocal color temperature. modeling light illumination that reveals the depth, shape, and texture of a subject; key light, cross lighting, counter-key light, side light, back light, and eye light are types of modeling light. modulation threshold in the case of sinisoidal wave gratings, manipulation of luminance differences can be specified in terms of modulation and the threshold can be called the modulation threshold. Periodic patterns that are not sinusoidal can be similarly specified in terms of the modulation of the fundamental sine wave component. The number of periods or cycles per degree of visual angle represents the spatial frequency. monocular visual field the field for a single eye. See binocular portion of the visual field. mounting height (roadway)† the vertical distance between the roadway surface and the center of the apparent light source of a luminaire. mounting height above the floor (MHf)† the distance from the floor to the light center of the luminaire, or to the plane of the ceiling for recessed equipment. mounting height above the workplane (MHwp)† the distance from the work plane to the light center of the luminaire, or to the plane of the ceiling for recessed equipment. multiple-beam headlamp a headlamp so designed as to permit the driver of a vehicle to use any one of two or more distributions of light on the road. Munsell chroma, C an index of perceived chroma of the object color defined in terms of the luminance factor (Y) and chromaticity coordinates (x, y) ''for CIE Standard Illuminant C and the CIE 1931 Standard Observer. Munsell color system a system of surface-color specification based on perceptually uniform color scales for the three variables: Munsell hue, Munsell value, and Munsell chroma. For an observer of normal color vision, adapted to daylight and viewing a specimen when illuminated by daylight and surrounded with a middle-gray to white background, the Munsell hue, value, and chroma of the color correlate well with the hue, lightness, and perceived chroma. Munsell hue, H the index of the hue of the perceived object color defined in terms of the luminance factor (''Y) and coordinates (x, y) ''for CIE Standard Illuminant C and the CIE 1931 Standard Observer. Munsell value, V the index of the lightness of the perceived object color defined in terms of the luminance factor ''Y ''for CIE Standard Illuminant C and the CIE 1931 Standard Observer. Note The exact definition gives Y as a fifth-power function of V, so that tabular or iterative methods are needed to find V as a function of ''Y. However, V can be estimated within ± 0.1 by V = 11.6(Y/100)1/3 − 1.6 or within ± 0.6 by V = ''Y''1/2, where ''Y ''is the luminance factor expressed in percent.